Discussions have existed for many years as to whether exhaust gas from automobiles with gasoline engines should be controlled because of problems with the global environment. In practice, the annual production of automobiles is still on the upswing but there is no prospect of reduction in automobile emissions. Under these circumstances, electric vehicles with batteries or solar cells have attracted attention as vehicles producing no exhaust gas. Therefore, there is an urgent need of early realization of practical electric vehicles.
In recent years, electric vehicles have begun to be used as vehicles in business applications such as urban-delivery vehicles and garbage trucks which are not required to travel a long distance continuously or to run at high speeds. Vehicles running at high speeds faster than 100 km/h and traveling about 200 km continuously have been reported as experimental vehicles. Furthermore, vehicles which have solar cells on the top of the body and run while charging the cells have been proposed. In addition, hybrid vehicles driven by both an internal combustion engine and an electric motor have been proposed.
One promising type of electric vehicle is a vehicle which has no gearing as used in an automobile with an internal combustion engine but drives the four wheels independently, using wheel motors. The driving mechanism of this vehicle is simplified. Also, the problems with the operating characteristics and the operability can be solved by coordinating and controlling the wheel motors. The greatest technical problem witch the electric vehicle is to realize an ideal power source, i.e., a battery having a capacity comparable to an automotive engine. In order to put the electric vehicle into practical use, a battery is needed which is comparable in size and weight to an internal combustion engine and whose capacity can deliver power comparable to the power delivered by a gasoline engine. Furthermore, the battery must be recharged quickly or must be replaced with a fully charged battery as simply as a supply of gasoline.
However, no conventional battery can satisfy the above-described requirements. One especially great problem is that it takes long for the prior art battery to be recharged. In spite of this fact, the prior art battery is larger in size and heavier than the internal combustion engine.
An electric double layer capacitor which is smaller in size but larger in capacitance than the prior art capacitor has been developed. This electric double layer capacitor tends to be used to back up a power supply or for other application. When a large-capacity capacitor such as this electric double layer capacitor is employed as a storage capacitor power supply, it has advantages in being lighter and having longer life than a lead-acid battery and other batteries. However, if the voltage applied to the electric double layer capacitor exceeds the rated voltage, then the capacitance of the capacitor is immediately reduced. Also, the leakage current increases. In this way, the capacitor is adversely affected. Another disadvantage is that the internal resistance and the maximum working voltage are not sufficiently controlled. For these reasons, positive use of the electric double layer capacitor for power use is not yet made.
Heretofore, when a secondary battery is recharged, various difficulties have arisen in precisely detecting the completion of the recharging or knowing how much electricity can still be obtained from the battery.
Various contrivances have been made to detect the completion of the recharging. One method is to set the end voltage at a given voltage. Another method is to estimate the completion from the amount of electricity flowed into the battery. A further method is to detect the instant at which the voltage slightly drops due to the temperature characteristics of the battery after it is recharged for a given time. In spite of these contrivances, the recharging characteristics vary widely according to the conditions of the battery, i.e., depending on whether the battery is new or old, on the extent to which the battery ages, on the recharging current, and on whether the battery has been used continuously or was recharged.
Also, various contrivances have been made to measure the remaining electric power. One method is to measure the terminal voltage while applying a given load. Another method is to calculate the amount from the amount of electricity charged and discharged. A further method is to estimate the amount from the temperature and the specific gravity of the electrolyte. However, the battery characteristics vary widely, depending on the performance of each individual battery, on whether the battery is new or old, on the history of the use of the battery, on the load, on the conditions imposed when the recharging is made, and on other factors.
As described above, it can be said that hardly any method of precisely detecting the full charge point at all times is available. Also, hardly any method of detecting the amount of the residual electric power in the battery exists. Furthermore, to make effective use of the electric power of the battery, it is customary to overcharge it to a given level. Also, it is essential to know the amount of the electricity remaining in the battery. In the future, electric vehicles will be put into practical use, and secondary batteries will be used routinely. Under these conditions, the wasteful consumption of electrical energy due to overcharge will present a problem that cannot be neglected. We may expect that the practicability of the battery depends on whether it is possible to know the distance that the vehicle can still travel.